Immunoassay and protein-binding procedures will be used to measure parathyroid peptides, calcitonin, vitamin D and vitamin D metabolites in biological fluids. These methods will be applied in studies of mineral metabolism in normally growing children and in children with a variety of disease states. Included will be studies during pregnancy and the perinatal period with emphasis on elucidating the factors involved in the pathogenesis of neonatal hypocalcemia. Studies will also be conducted in children with renal insufficiency, growth hormone deficiency, and metabolic bone disease. We will continue and extend studies that relate to the interrelationship between magnesium metabolism and parathyroid function and calcitonin. In vivo and in vitro experimental animal studies will complement clinical investigations. Laboratory methods in these studies will include immunoassay determinations of serum parathyroid peptides and calcitonin, measurements of urinary cyclic AMP and hydroxyproline, and determinations of serum 25-hydroxycholecalciferol, 1,25-hydroxycholecalciferol, and ionized calcium activity of serum and urinary calcium, magnesium and phosphorus.